Embryos of the frog, X. laevis are well studied by embryologists and have long been used as a toxicological model. The zebrafish, D. rerio on the other hand, is a more genetically tractable model than Xenopus for some mammalian developmental processes (e.g., neural tube closure). A third species, Xenopus tropicalis (X. tropicalis) is a cousin of X. laevis, is suitable for genetics but is novel as an experimental species. This proposal aims to compare these three species in their toxicological responses and to apply molecular approaches to define targets of select toxicants. Nicotine and valproic acid are the chosen toxicants because of their known effects in X. laevis and their potential clinical relevance (nicotine to pregnant women attempting to quit smoking and Valproic acid because it causes an autism-like syndrome). There are candidate gene targets for both toxicants, the Pax-6 and Hox1a transcription factors, respectively. Detailed anatomical analysis will be carried out to establish the effective doses, the period of sensitivity and morphological effects of each toxicant in X. laevis, X. tropicalis and zebrafish. A medium-scale screen will be carried out in X. laevis and X. tropicalis for toxicant modulation of marker gene expression. Many marker genes are available from a database of X. laevis in situ expression patterns. This will establish the earliest detectable perturbations, identify biomarkers of potential practical use, and may also give clues to the proximal target and mechanism of action of the toxicants. Selective genes whose action is perturbed will be targeted specifically using antisense morpholino oligonucleotides. The phenotypic effects of gene- specific interference will be compared in detail with toxicant effects to establish or exclude these genes as likely targets of toxicant action. Finally, X. laevis microarrays will be used to identify further immediate- early and highly-responding transcriptional targets of these model toxicants for future investigation. These experiments will generate useful comparative information as well as lay the foundation for illuminating the mechanisms of developmental toxicity at the molecular and genomic levels.